Golden opportunity

Swiss researchers believe they can explain the behaviour of gold particles, which could pave the way to new applications in fuel cells and air purification. Stuart Nathan reports.

Gold is a bit of a puzzle to engineers and scientists. Its reluctance to form compounds -the very thing that has made it so precious for jewellery and art – renders it useless for chemicals; but its high electrical conductance makes it invaluable in electronics.

Most confusing of all, despite its chemical inertness, is its strong activity as a catalyst – but only when in the form of small particles. Researchers at the European Synchrotron Radiation Facility (ESRF) in Grenoble believe they can explain this property – which could lead the way to new applications in fuel cells and air purification.

Gold nanoparticles are able to catalyse the oxidation of carbon monoxide to carbon dioxide, but the activity depends very strongly on the size of the particles; as the size increases, the activity tails off rapidly. The research, led by Jeroen van Bokhoven of ETH Zurich‘s Institute for Chemical and Bio-Engineering, decided to use the ESRF’s high-energy beamline to study the reactions.

The beamline allowed the researchers to use X-ray absorption spectroscopy to study how the oxygen in CO behaves when adsorbed on to the surface of the gold nanoparticles. Because of the power of the beam they could observe the different species being formed on the surface of the gold particle.

First, they pumped oxygen over the surface of the gold, and observed how it bound to the surface and became more reactive. Then, they switched to carbon monoxide, and noticed that the CO molecules reacted with the oxygen already on the surface.

The key to the reaction seems to be a change in the nature of the gold itself – it becomes partially reduced, meaning that it has spare electrons which can be passed to atoms on its surface.

When oxygen is bound to the surface of the gold, these electrons can pass to the oxygen atoms, which makes them extremely reactive. This allows them to latch on to carbon monoxide, forming carbon dioxide molecules on the gold surface. These then float free, allowing the process to begin again.

The reason that the size of the particles matters, explained van Bokhoven, is that extremely small ones have a very different arrangement of electrons than larger chunks.

Atoms in metals tend to stack up, like layers of oranges in a box, with their electrons forming a kind of electrostatic glue that holds the structure together. Atoms on the edge of a stack have more free electrons than those in the middle. The smaller the particle, the more of these stack-edge atoms there are.

Moreover, these free electrons tend to be quite loosely bound to the atom. This means that, first, nanoparticles tend to adsorb gas molecules -binding them to the surface -more strongly than larger particles; and they also tend to give the electrons away more readily.

‘We knew beforehand that small gold particles were active, but not how they did the reaction,’ said van Bokhoven. ‘The good thing is that, for the first time, we have been able to observe the steps and path of the reaction. The results almost perfectly followed our original hypothesis. Isn’t it beautiful that the most inert bulk metal is so reactive when finely dispersed?’

Possible applications of the research include pollution control, particularly air cleaning, and purification of hydrogen streams for fuel cells. One of the most common methods of producing hydrogen for fuel cells is by reforming methane, which produces a mixed stream of hydrogen and carbon monoxide; the gold method could be extremely useful in removing the CO.

The ESRF system could also become useful, as it has now been proven as a way of studying catalysis as it happens. ‘The exceptionally high structural detail that can be obtained with it could be used to study other catalytic systems, with the aim of making them more stable and perform better,’ he said.